Optimization of L-band sea surface emissivity models deduced from SMOS data

X. Yin(1), J. Boutin(1), N. Martin(1), P. Spurgeon(2)

(1) LOCEAN, Paris, France (2) ARGANS, Plymouth, UK

Two scale + foam

Adjustment of some parameters of roughness and foam modeling

100.225log ( /2)23 *

0*

1.25( )

kku

S k a kg

Roughness:Omnidirectional wave spectrum Durden & Vesecki,1985 :

Foam coverage (from Monahan & O'Muircheartaigh 1986): 10 exp( 0.0861 )cF bU T

a0? Original publication: a0=0.004; DV2, a0=0.008

b? c? original publication: b=1.95×10-5, c=2.55 ; ΔT =Tsea-Tair (neglected in this first step study); in first SMOS SSS1 processing, F=0: no foam.

Foam emissivity (Stogryn, 1972): assumed to be correct

~0.2K/m/s

Dinnat et al., IJRS, 2002, Radio Science, 2003

At 15°C, a 0.1K Tb variation can be generated by :

-0.2pss SSS variation or

- 0.5m/s wind speed variation

10m equivalent neutral wind speed (m/s)

Nadir

Th_30°

DV2

Modeling of SMOS Tbs• Tb = Tbatm↑ + Rsea (Tbatm↓ + Tbsky) exp(-atm) + Tbsea exp(-atm)

Ocean

Atmosphere

Tbsea= (Tbflat+Tbrough) (1-F) + F Tbfoam

esea derived from SMOS Tbs after correcting for all other effects

Tbsea=esea SST

Incidence angle (°)Radiometric accuracy Along track in the AFFOV

0

5K

0

5K

SMOS Tbs: Tbs along track (~ no mixing of polarization) in the AFFOV (good radiometric accuracy) from 19 ascending orbits in August (low galactic noise) in the South Pacific (far from land) from 50°S to 0°N –

Incidence angles from 20° to 55°

SMOS data used in the fit

Three different sets of wind induced components deduced from SMOS

1. There are totally 237501 samples in H polarization and 238469 in V polarization collocated with ECMWF WS in range of 3-17ms-1.

2. 56% of the ECMWF wind speeds (136861 samples in H polarization and 137471 in V polarization) have been collocated with SSMI WS in range of 3-17 ms-1 : +-0.5h +-50km

3. 126662 samples in H polarization and 127191 in V polarization, when the differences between ECMWF and SSMI WS were restricted to be less than 2 ms-1

3m/s<U<7m/sa0 (prior=0.004 – 0.008)

8m/s<U<17m/sb, c

Data fitting

Wind induced component of emissivity deduced

Er_SMOS (θi, p, ws)= Eres (θi, p) + Espectrum(a0; θi, p, ws)

(20-55° in step of 5°)

Incidence angle (°)

a0 b c

M1 0.0050 2.42×10-8 4.86

M2 0.0062 2.20×10-9 5.67

M3 0.0070 2.90×10-9 5.51

M1 M2

M3

ECMWF ECMWF+SSMI

SSMI ECMWF

Results in AFFOV

H pol. 20° V pol. 20°

H pol. 30° V pol. 30°

H pol. 40° V pol. 40°

H pol. 50° V pol. 50°

H pol. 55° V pol. 55°

ECMWF WS ECMWF WS

Results in EAFFOV ?H pol. 0° V pol. 0°

V pol. 10° V pol. 20°

Comparisons w.r.t WOA05

Old model 1 (DV2)

New parametrization for roughness and foam coverage

Monthly averages, Pacific Ocean,August 2010

SSS North-South profile,

1. The tropical Southern Pacific ocean (20°S10°S- 140°W110°W) far away from continent and island characterized by relative stable moderate wind speed and high SST; mean (standard deviation) of SST and SSS are 24.5 (1.0) °C and 36.2 (0.3) pss

2. The high latitude Southern Pacific ocean (50°S45°S- 180°W100°W) characterized by very variable wind speed and low SST; mean (standard deviation) of SST and SSS are 9,8 (1.8) °C and 34,4 (0.2) pss

SMOS SSS retrieved with the pre-launch model 1

SMOS SSS with the new model M1

in red for the tropical Southern Pacific and in green for the high latitude Southern Pacific

Summary

• SMOS data evidence that Tb(U) is non linear• A reasonnable fit to SMOS data is obtained when introducing a foam

coverage parametrization close to Monahan and Muircheartaigh (1986), (this foam coverage may be peculiar to L-band and depends on the foam emissivity model)

• Parameter for the wave spectrum (a0) slightly higher than 0.004• Preliminary validation shows improvement in mean retrieved SSS

• A larger set of SMOS data should be used for validating and/or improving model

• Study the quality of SSS retrieved at high wind speed when putting a larger error on ECMWF wind speed.

• Check SMOS measurements taken in the EAFFOV

Latitudinal drift ?EH_SMOS - EH_Model, 0deg

EV_SMOS - EV_Model, 0deg

Zone Southern PacificLatitude 50S-40S 20S-10SLongitude 180W-100W 140W-110WSST (°C) 9.8 24.5σSST (°C) 1.8 1.0SSS (pss) 34.4 36.2σSSS (pss) 0.2 0.3

pre-launch model 1Wind speed (ms-1) 3-12 12-20 3-12No. of collocations 11714 4060 4556mean(SSSsmos-SSSargo) (pss) 0.08499 -0.89729 0.08477σ (SSSsmos-SSSargo) (pss) 1.06832 1.37645 0.62445median(SSSsmos-SSSargo) (pss) 0.1 -0.82515 0.0905

New model M1No. of collocations 11718 4150 4551mean(SSSsmos-SSSargo) (pss) -0.04846 -0.40917 -0.06231σ (SSSsmos-SSSargo) (pss) 1.0703 1.50764 0.63529median(SSSsmos-SSSargo) (pss) -0.0268 -0.5132 -0.055

Statistics of SMOS SSS collocated with ARGO at +/-5days and +/-50km during August ascending passes